s forward the puppet _p_, in which the
graver of chisel _g_ slides _without shake_. This graver has a point
or edge shaped properly to form the spiral groove, with a shoulder to
regulate the depth of the groove. The iron rod _r_, which is firmly
fastened in the puppet, slides through mortices at _mm_, and guides
the puppet in a straight line.

[Illustration: Plate 1.]

The rest of the machine is intelligible from the drawing.

A simple method of showing the nature of compound forces was thought
of at the same time. An ivory ball was placed at the corner of a board
sixteen inches broad, and two feet long; two other similar balls were
let fall down inclined troughs against the first ball in different
directions, but at the same time. One fell in a direction parallel to
the length of the board; the other ball fell back in a direction
parallel to its breadth. By raising the troughs, such a force was
communicated to each of the falling balls, as was sufficient to drive
the ball that was at rest to that side or end of the board which was
opposite, or at right angles, to the line of its motion.

When both balls were let fall together, they drove the ball that was
at rest diagonally, so as to reach the opposite corner. If the same
board were placed as an inclined plane, at an angle of five or six
degrees, a ball placed at one of its uppermost corners, would fall
with an accelerated motion in a direct line; but if another ball were
made (by descending through an inclined trough) to strike the first
ball at right angles to the line of its former descent, at the moment
when it began to descend, it would not, as in the former experiment,
move diagonally, but would describe a curve.

The reason why it describes a curve, and why that curve is not
circular, was easily understood. Children who are thus induced to
invent machines or apparatus for explaining and demonstrating the laws
of mechanism, not only fix indelibly those laws in their own minds,
but enlarge their powers of invention, and preserve a certain
originality of thought, which leads to new discoveries.

We therefore strongly recommend it to teachers, to use as few precepts
as possible in the rudiments of science, and to encourage their pupils
to use their own understandings as they advance. In mechanism, a
general view of the powers and uses of engines is all that need be
taught; where more is necessary, such a foundation, with the
assistance of good books, and the